1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive (MR) sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP-MR sensor with a more stable reference layer.
2. Background of the Invention
One type of conventional MR sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer adjacent the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference layer. The other ferromagnetic layer adjacent the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the reference-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
In addition to CPP-GMR read heads, another type of CPP-MR sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. In a CPP-GMR read head the nonmagnetic spacer layer is formed of an electrically conductive material, typically a metal such as Cu. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO2, MgO, or Al2O3.
The fixed or pinned ferromagnetic layer in a CPP-MR sensor used in read heads may be a single pinned layer (sometimes called a “simple” pinned layer) or part of an antiparallel (AP) pinned structure. In a simple pinned structure the pinned layer may have its magnetization pinned by being exchange-coupled to an antiferromagnetic pinning layer, which is typically a Mn alloy like IrMn. In an AP-pinned structure first (AP1) and second (AP2) ferromagnetic layers are separated by a nonmagnetic antiparallel coupling (APC) layer with the magnetization directions of the two AP-pinned ferromagnetic layers oriented substantially antiparallel. The AP1 layer, which is in contact with the antiferromagnetic layer on one side and the nonmagnetic APC layer on the other side, is the pinned layer. The AP2 layer, which is in contact with the nonmagnetic APC layer on one side and the sensor's electrically conductive spacer layer on the other side, is the reference layer.
Due to decreasing read head dimensions, the stability of the reference layer has become an issue. One technique that addresses this problem is directional ion milling of a seed layer onto which the reference layer is subsequently deposited. This results in additional uniaxial anisotropy in the reference layer. This technique is described in U.S. Pat. No. 7,564,659 B2 and U.S. Pat. No. 7,762,090 B2, both assigned to the same assignee as this application. However, with this technique it can be difficult to achieve uniformity over an entire wafer, from which a large number of sensors are fabricated.
What is needed is a CPP-MR sensor with a reference layer with induced uniaxial anisotropy that will improve stability.